Meaning & Usage
The Japanese grammar pattern ことにする (koto ni suru) is used to express that someone has made a personal decision or resolution. This often involves choosing an action after some thought or deliberation, frequently establishing it as a personal rule or plan. The decision is typically internal, made by the speaker or the subject of the sentence, rather than being imposed by external factors.
You can think of ことにする as meaning, "I have decided to do X," or "I am making it a point to do X." It strongly emphasizes the act of making a deliberate choice and committing to it. This can be a one-time decision or a recurring habit that the speaker has personally established. For example, if you've been considering improving your health and finally commit to exercising every day, you would use ことにする to express that resolution: 「明日から毎日ジョギングすることにする。」(Ashita kara mainichi jogingu suru koto ni suru. - I've decided to jog every day starting tomorrow.)
In English, we might simply say "I decided to do X." While similar, ことにする carries a slightly stronger nuance of intention and personal commitment in Japanese. It's more than a general decision; it's one you are personally undertaking or adopting. It differs from simply stating a plan (like with ～つもりだ, which expresses intention but not necessarily a firm, enacted decision) or a future event (like with plain dictionary form verbs). The inclusion of こと, meaning "thing" or "matter," effectively turns the verb into a noun phrase, which is then "made into" (にする) a decision or custom.
This pattern is commonly used in both spoken and written Japanese. While applicable in various contexts, it primarily reflects a conscious, deliberate choice. It's a fundamental N4 grammar point, enabling speakers to clearly articulate their personal resolve and future actions based on that resolve.
Structure & Formation
The structure of ことにする is quite straightforward. It primarily attaches to the plain form of verbs, allowing you to express a decision about an action. Here are the main patterns:
| Type | Structure | Meaning |
|---|---|---|
| Affirmative Decision | Verb (dictionary form) + ことにする | To decide to do V |
| Negative Decision | Verb (ない form) + ことにする | To decide not to do V |
Formation Examples:
- 行く (iku) → 行くことにする (iku koto ni suru) — To decide to go
- 食べない (tabenai) → 食べないことにする (tabenai koto ni suru) — To decide not to eat
- 買う (kau) → 買うことにした (kau koto ni shita) — Decided to buy (past tense)
- 買わない (kawanai) → 買わないことにした (kawanai koto ni shita) — Decided not to buy (past tense)
The tense of ことにする changes with the conjugation of する (suru). So, ことにします is the polite present/future form, and ことにしました is the polite past form.
Example Sentences
Daily Decisions & Plans
明日からジョギングを始めることにした。
Ashita kara jogingu o hajimeru koto ni shita.
I decided to start jogging from tomorrow.
今日は早く寝ることにする。
Kyō wa hayaku neru koto ni suru.
I've decided to go to bed early today.
週末は家でゆっくりすることにします。
Shūmatsu wa ie de yukkuri suru koto ni shimasu.
I've decided to relax at home this weekend.
携帯を新しいものに替えることにした。
Keitai o atarashii mono ni kaeru koto ni shita.
I decided to switch to a new mobile phone.
Negative Decisions
もうタバコは吸わないことにする。
Mō tabako wa suwanai koto ni suru.
I've decided not to smoke anymore.
今夜は外食しないことにしました。
Kon'ya wa gaishoku shinai koto ni shimashita.
I decided not to eat out tonight.
このプロジェクトは引き受けないことにする。
Kono purojekuto wa hikiukenai koto ni suru.
I've decided not to take on this project.
ダイエットのため、お菓子は買わないことにしています。
Daietto no tame, okashi wa kawanai koto ni shite imasu.
For my diet, I've made it a rule not to buy sweets.
Decisions for Others (or imposed)
彼は来年、アメリカへ留学することにしたそうだ。
Kare wa rainen, Amerika e ryūgaku suru koto ni shita sō da.
I hear he decided to study abroad in America next year.
会議は来週に延期することにしました。
Kaigi wa raishū ni enki suru koto ni shimashita.
We decided to postpone the meeting until next week.
私たちは結婚しないことにした。
Watashitachi wa kekkon shinai koto ni shita.
We decided not to get married.
彼がリーダーを務めることに決まった。
Kare ga rīdā o tsutomeru koto ni kimatta.
It was decided that he would serve as leader. (Here, ことに決まる is used for a decision made by a group or external force)
Common Mistakes
Mistake 1: Confusing with ～つもりだ (tsumori da)
❌ 明日、旅行に行くつもりだ。
✅ 明日、旅行に行くことにした。
～つもりだ expresses an intention or expectation, similar to "I intend to go" or "I plan to go." It doesn't necessarily imply a firm, enacted decision or resolution. In contrast, ことにする conveys that a concrete decision has been made and is now a settled plan for the speaker. If the decision is firm and you've made up your mind, ことにする is more appropriate.
Mistake 2: Confusing with ～ことにしている (koto ni shite iru)
❌ 毎日、運動することにした。
✅ 毎日、運動することにしている。
While ことにする denotes a one-time decision or a decision about a future action, ことにしている implies that a decision has been made and is now a continuous habit, custom, or personal rule. If it's something you do regularly as a result of a past decision, the ～ている form is necessary. For example, 「毎日、運動することにしています。」(Mainichi, undou suru koto ni shite imasu. - I make it a rule to exercise every day.)
Mistake 3: Using with external decisions (without context)
❌ 会社が新しいシステムを導入することにした。
✅ 会社は新しいシステムを導入することに決まった。
ことにする primarily describes a personal decision made by the subject. When a decision is made by a group, an organization, or an external force, it's more natural to use ～ことに決まる (koto ni kimaru), meaning "it has been decided," or to phrase it more passively. While you can use ことにした if the speaker is part of the group making the decision, it's less direct for truly external decisions. For example, a company announcement would use 「導入することに決まりました。」
Mistake 4: Incorrect Verb Form
❌ 買いたいことにする。
✅ 買うことにする。
ことにする must always attach to the plain dictionary form (or plain negative form) of a verb. It cannot attach to volitional forms like ～たい (tai) or other conjugated forms. Always revert the verb to its dictionary or ない form before adding ことにする.
Cultural Notes
In Japanese culture, decision-making often involves a degree of consideration for others, even when the decision is personal. While ことにする expresses personal resolve, the context in which it's announced or acted upon might still reflect politeness or an awareness of social harmony. For example, when making a personal decision that impacts others (like changing plans), one might preface it with an apology or an explanation, such as 「申し訳ありませんが、参加しないことにしました。」 (Moushiwake arimasen ga, sanka shinai koto ni shimashita. - I'm sorry, but I've decided not to participate.)
Furthermore, the act of making a firm decision (as implied by ことにする) is often respected. When someone uses this phrase, it typically signals that they have weighed their options and are committed to a particular course of action. This implies reliability and seriousness about their intentions, contrasting with more tentative expressions of desire or possibility. It's a clear statement of agency and personal choice.
JLPT Tips
For N4 learners, understanding ことにする is crucial for clearly expressing personal decisions and plans. Here are some tips:
Distinguish from Similar Grammar: Pay close attention to the nuanced differences between ことにする, ～つもりだ, and ～ことにしている. While all relate to future actions or intentions, ことにする emphasizes a firm, conscious decision. Remember, ～つもりだ is about intention, and ～ことにしている is about established habits.
Verb Form is Key: Always remember that ことにする attaches to the plain dictionary form (for affirmative) or plain negative form (for negative) of the verb. Incorrect conjugation is a common error in exams.
Context Matters: Practice identifying sentences where the speaker is making a personal choice or resolution. If the decision is by a group or an external entity, or if it's a statement of fact rather than a personal commitment, ことにする might not be the best fit.
Practice with Example Sentences: Create your own sentences using everyday scenarios. This helps solidify your understanding and makes it easier to recall during tests. Focus on scenarios where you actively decide to do or not do something.
Look for Keywords: In listening or reading comprehension, words like 「決める (kimeru - to decide)」 or 「考える (kangaeru - to think/consider)」 often precede the decision-making process that ことにする expresses.